The development of semiconductor variable frequency sources, such as inverters and cyclo-converters, has allowed the use of synchronous motors in a wide variety of applications. The high efficiency of synchronous motors makes them more appealing than induction motors. Commonly used synchronous motors include wound field synchronous motors (WFSM), permanent magnet synchronous motors (PMSM), synchronous reluctance synchronous motors (SRSM), and hysteresis motors.
Drive systems are used to operate these motors. The drive system is responsible for operating the motor in the forward and reverse direction, and for the acceleration and braking of the motor as well.
Synchronous machine drive systems generally comprise a synchronous machine and an inverter that is configured from a plurality of pairs of solid state switching devices, typically silicon controlled rectifiers (thyristors). Rendering the thyristors conductive in a predetermined control sequence causes the synchronous machine to be supplied with alternating current and hence to commence the appropriate rotation.
Conventionally, WFSM has become the widely used drive for high-power applications in the range of a few MW's to 100s of MW's. There are most commonly two drive configurations for driving a WFSM at high-power, including the cyclo-converter and the load commutated inverter (LCI), each configuration generally being used for different applications. The application areas for these two drive methods are typically divided based on the operating speed range of the WFSM.
In low speed applications, such as rolling mills, ball mills, mine hoists and ship propulsion, naturally commutated cyclo-converter driven WFSM drives are widely used. In this speed range, the cyclo-converter driven WFSM provides a high performance drive solution with the ability to operate in all four quadrants of the plane whose axes are torque and speed (four-quadrant operation), indicating an ability to forward and reverse brake, and forward and reverse motor.
In normal to high speed applications, such as fans, compressors, and pumped storage plants, load commutated inverter (LCI)-fed WFSM drives are the most widely used solution. The LCI drive method is preferred in high speed applications due to the ability to decouple the operation of the rectifier and inverter. Additionally, the LCI typically has two bridges, compared to a cyclo-converter that requires six bridges for the typical six-pulse configuration.